Virus entry is an attractive new target for anti-viral treatment, and about 10 drugs that are designed to block virus attachment or membrane fusion are currently being evaluated in preclinical or clinical studies (Richman, 1998; PhRMA, 1999; Stephenson, 1999). Enveloped animal viruses attach to and enter the host cell via the interaction of viral proteins in the virion membrane (envelope proteins) and cell surface proteins (virus receptors). Receptor recognition and binding are mediated by the surface envelope protein.